NATURAL SCIENCES OF PHILADELPHIA. 147 



size ; last one forming about one-third the entire length, rounded, not much 

 produced below ; suture well defined ; aperture oval subrhombic, slightly 

 effuse on the inner side below ; outer lip thin, and nearly straight ; inner lip 

 a little reflexed. Surface ornamented by small, regular, straight, vertical 

 folds or costae, about equalling the spaces between, and numbering near thirty 

 on the body whorl. Costae obsolete on the under side of the last turn : no 

 lines of growth visible under a lens. 



Length, 0*36 inch ; breadth, 0-15 inch ; apical angle nearly regular, diver- 

 gence about 28. 



Locality and position. Hodges Creek, Macoupin Co., 111. Coal Measures. 



CEPHALOPODA. 

 Genus ORTHOCERAS, Breynius, 1732. 



Orthoceras annulo-costatum. Shell attaining a medium size, and having 

 the form of an elongated, moderately compressed cone, the sides of which 

 converge towards the apex at an angle of about 14. Section elliptical, the 

 greater transverse diameter being to the smaller, as 100 to 80. Surface or- 

 namented with slightly oblique, annular costa?, which are less than the de- 

 pressions between, and rather sharply elevated on the smaller half of the 

 shell, but become gradually obsolete towards the aperture, where they are no 

 more widely separated than near the smaller end. Traces of fine transverse 

 striae are also seen on well preserved specimens, both between and upon the 

 costae. (Septa and siphuncle unknown.) 



The largest specimen we have seen is about five inches in length, (both ex- 

 tremities being incomplete,) and 1*83 inches in its greater diameter at the 

 larger end, while the greater diameter at the smaller end is near 0*70 inch. 



This species bears some resemblance to 0. dactylio-phorum of De Koninck, 

 (An. Fos. Ter. Carb. Belg. p. 518, PL XVII. fig. 1 A, and XVIII. fig. 7, a, b,) 

 but differ in being more tapering and somewhat compressed instead of round. 



Locality and position. Chester, 111. Chester Limestone. 



Genus NAUTILUS, Breynius, 1732. 

 Subgenus Trematodiscus, M. & W. 



(rpi/u* and (fr;w?,) in allusion to the perforated umbilicus, and the discoid 

 form of the shell in the typical species. 



We propose to range under the above subgeneric name, a peculiar group of 

 Nautili, which differ so materially from the living typical species of Nautilus, 

 that few Conchologists would place them even in the same genus, if they 

 were found inhabiting our present seas. These shells are characterized by a 

 discoid form and a wide shallow umbilicus, usually, if not always, perforated 

 in the middle. Their whorls are slender, very slightly embracing or merely in 

 contact, and provided with revolving angles and grooves, while the surface is 

 frequently ornamented with revolving striae. The siphuncle is central, or 

 located between the middle and the dorsal side, though never quite marginal. 



In first publishing descriptions of some of the forms included in this sub- 

 genus, we proposed to retain for it Prof. King's abandoned name, Discus. As 

 this name had, however, been previously used by Fitzinger (in 1833) for a 

 group of Hr-licidse, and by Prof. Haldemau (in 1840) for a group of Limna- 

 idce, it becomes necessary to introduce a new name for the group under consid- 

 eration. 



Although in some respects analogous to Discites of McCoy, these shells differ 



1861.] 



